Born with a rare and unfortunate genetic mutation that made his skin green and scaly and grew his body to grotesque proportions, Waylon Jones was raised by his alcoholic aunt and bullied relentless for his appearance.

For a time, he worked as a carnival freak and was named Killer Croc, but his misanthropy increased along with his animalistic nature, and he fell into a life of crime. As his physical condition and mental state deteriorate, Killer Croc becomes a more bestial foe, increasingly detached from humanity.

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On Christmas Eve, Killer Croc was hired by the Joker, posing as Black Mask, to kill Batman. Croc followed Joker through Blackgate Prison, witnessing the freeing of Calendar Man and murder of Commissioner Gillian Loeb. Catching Batman's scent, Croc decided to stay on the roof of the prison instead of joining the other henchmen via helicopter. After defeating Croc, Batman pinned him to a gate and interrogated him. Croc revealed that he wasn't the only assassin hired that night. After being knocked unconscious, Killer Croc was returned to his cell.[1]

During Joker's riot later that night, Croc escaped his cell and tracked down Batman. As Croc picked up Batman, a sniper had his shot trained on Batman. James Gordon snuck up behind the sniper and knocked him out, causing his shot to misfire and graze Croc's shoulder. Croc dropped Batman and told him he could have Joker, seeing the contract more trouble than its worth.[1]

Several years later, Batman brought Joker into Arkham Asylum again. While Joker was escorted to his cell, Killer Croc was being brought up via elevator. After exiting the elevator, Croc caught Batman's scent and stated he would hunt him down. The guards activated his shock collar, forcing Croc to comply and continue walking.[2]

After Joker's coup, Batman made his way to the Batcave to analyze Penelope Young's Titan formula. After exiting the Batcave, Batman explored the sewers and found a door leading to Croc's lair. Realizing the door was locked, Batman stood back as Killer Croc smashed the door window, once again threatening to kill Batman.[2]

Later that night, Batman attempted to make a cure to the Titan. Poison Ivy told Batman of a plant that could help, but it only grew in Killer Croc's later. As Batman made his way to Croc's lair, he was ambushed by Scarecrow's fear toxin. Overcoming the toxin, Batman chased Scarecrow into the sewers. Scarecrow threatened to drop his toxin into the water supply, spreading fear all over Gotham City. At that moment, Killer Croc leaped out of the water and grabbed Scarecrow. Batman attempted to pacify him by throwing a Batarang at Croc's shock collar. Temporarily stunned, Croc jumped back into the water with Scarecrow in tow.[2]

While exploring Croc's lair for Ivy's plant spores, Batman was continuously attacked by Croc, only to be shocked by his collar and forced to retreat. After collecting all the spores, Batman made his way back to the lair's entrance. This caught Croc's attention, chasing Batman to the entrance. Seemingly cornered, Batman lured Killer Croc into a trap he had set earlier with Explosive Gel. Croc survived the fall and swore he would find Batman.[2]

After Batman acquired a sample of Rā's al Ghūl's blood, he made his way through the sewers, looking for an exit to the surface. Seeing his path blocked, Batman threw a Remote Controlled Batarang at a switch. This alerted Killer Croc, who surfaced from one of the underwater tunnels and surprised Batman. Croc smelled a different scent on Batman, that of death. He decided not to fight Batman, instead choosing to wait for Batman to die and retrieve the body later.[3]

Late into Halloween, the combination of Scarecrow's toxin and Joker's blood forced Batman into a hallucinogenic state. Believing himself to be Joker, Batman was tied up and held hostage by several other supervillains, including Killer Croc. After telling Croc to duck, Joker called his "Jokermobile", knocking Croc out upon arrival.[4]

After Arkham City shut down, Killer Croc was tracked to his den, captured and imprisoned in the prison airship, Iron Heights Penitentiary. The warden, Ranken, experimented on Croc, transforming his body due to trauma. While over Gotham airspace, Croc escaped his cell and ripped apart the engine, causing the airship to crash down. After Batman arrived on the scene, he rescued security guards and the warden. Croc abducted the warden and took him deeper into the prison.[5]

After obtaining two key-cards, Batman and Nightwing followed Croc, learning from video footage that Ranken was experimenting on Croc and other inmates, hoping to weaponize Jones' condition. Nightwing and Batman fought Croc and his lackeys, eventually knocking him unconscious. Batman freed the warden, only to knock him out as well for what he did to Croc and the other prisoners. Batman used the Batwing to airlift Croc and Ranken to the GCPD, locking them up next to each other.[5]